According to Tommaso Checchi, a developer at Mojang, the Mojang team had begun porting Minecraft to Windows Phone before the Microsoft acquisition was announced, though work has not progressed too far. He tweeted on the matter, saying, "The port is in the first stages though. It's kind of slow now and needs optimization, so in full Mojang style the release date is "soon"™. And nobody forced/threatened us if you're wondering!" Even better, Minecraft is expected to be optimized to run on lower-spec Windows Phone devices whenever it is released, which means the entire WP ecosystem will be able to handle the game.
When it was announced that Microsoft would be buying Mojang, its creator Markus Persson announced that he would be leaving Mojang behind to focus on his real passion, which is making games. He said that he had no interest in managing a behemoth product like Minecraft. Persson had also been critical of Windows platforms in the past, making it more of a surprise that he decided to sell to Microsoft and that work had begun on the Windows Phone port of Minecraft before Persson officially left.
No word on when we'll see Minecraft on Windows Phone, but it is definitely in the works. We'll let you know when there's more info on the topic.